no low end st70

As the Les McCann and Eddie Harris song goes, "Compared to What?" What amp gave you better bass response and what preamp are you using? The Synthesis speakers were designed on tubes, so this should not be an issue. Oh, you said that you are using the Triode boards, which brand of coupleing caps are you using in them?

I think Trelja pretty much hits all of the points. Especially the power issues and the impedence matching ones as well. If your amp is biased properly (and this is very important) and you still have poor bass response, you may have to just consider getting either more suitible speakers or a different amp. I've had more success in the bass department as well as FR linearity, using amps utilizing the 6550/KT88/KT90 family of tubes.

Atmasphere (Ralph) raises a good point. Certainly worth checking this out. Years ago, I had a pair of speakers (Rectilinear III's) that came from the factory wired out of phase! In checking the phase, if it seems correct, try reversing polarity of only one of the speakers just for the heck of it and see what happens (just in case the speakers are incorrectly internally wired).